A conventional receiver circuit to conduct TDMA receiving (herein, also referred to as `time division multiplex receiving`) comprises a receive mixer circuit, an IR (intermediate-frequency) filter, an IF amplifier, a demodulator, and a controller to which a field strength output from the IF amplifier is input. This is a typical receiving circuit in which one IF filter is provided for one intermediate frequency.
For example, TDMA transmitting and receiving are conducted by using four transmit slots, T1, T2, T3 and T4 followed by four receive slots, R1, R2, R3 and R4. The four transmit slots and four receive slots compose one frame length. The TDMA transmitting and receiving are conducted by repeating such frames.
With regard to the four receive slots, in typical cases, one slot is assigned to a control channel and the other three slots are assigned to call channels. Thus, three calls at maximum can be simultaneously used.
Also, whether a receive slot is vacant and can be used as a call channel is judged as follows. In general, the field strength output from the IF amplifier is compared with a predetermined threshold value in the controller. When the former is higher than the latter, it is judged that an electric field exists and the receive slot is occupied. On the other hand, when the former is lower than the latter, it is judged that no electric field exists and the receive slot is vacant.
Thus, call channels can be in turn assigned to vacant slots, thereby allowing three calls at maximum to be used.
However, in the conventional TDMA receiver circuit, there is the problem that a receive slot to be duly judged as having no received field is erroneously judged as having a received field due to a delay in fall time of the field strength output for the previous receive slot. Therefore, the receive slot is not available for a call channel, thereby the three calls at maximum cannot be obtained and the connection rate must be reduced.
The problem is frequently caused by the performance of a SAW filter typically used as the IF filter.
Thus, it may happen that a receive slot duly available for a call channel cannot be used. This causes a reduction in the connection rate.
Especially when a received field to be input is high, the erroneous judgment as having a received field may frequently happen because the delay in field detection up to the next slot becomes significant with an increase in field strength level.